1. Field of the Invention
This invention relates to crosslinkable polymeric compositions which exhibit moisture and heat resistance and flame resistance and which are useful in producing insulated wire and cable as well as molded products. More particularly, it relates to a crosslinkable ethylene-vinyl acetate copolymer composition capable of passing a test procedure known as the CSA Varnish Test.
2. Description of the Prior Art
One of the most important areas where fire resistant polymer compositions find use is in the electrical environment, i.e., where both insulating and fire resistant properties are sought, most especially in the area of conductor insulation. At one time, extrudable compositions available to the wire and cable art were required, for flame resistance, to contain halogenated polymers such as chlorinated polyethylene, polyvinyl chloride, chlorobutadiene, chlorinated paraffin, etc., together with antimony trioxide, both components being present in sizable quantities. Alternatively, a coating of chlorosulfonated polyethylene paint was applied to a nonflame retardant insulating compound which constituted an additional manufacturing operation.
For certain types of dry transformers, particularly high voltage transformers, a problem existed in that electrical failures occurred due to surface creepage of the organic insulating component used. The problem was solved through the addition of hydrated alumina to compositions whose organic binder consisted of butyl rubber, epoxy resins or polyester resins. However, these compositions do not possess a balance of excellent extrudability characteristics, physical and electrical properties, heat resistance and flame retardance. Such compositions are disclosed in U.S. Pat. Nos. 2,997,526-7 and 8 of Kessel et al. The described compositions for such usage have poor tensile strength, elongation and percent elongation retained after aging.
Fire retarding polymeric compositions exhibiting, inter alia, improved moisture and heat resistance consist essentially of an intimate mixture of at least one cross-linkable polymer containing as a major component an ethylene-vinyl acetate copolymer, one or more silanes and one or more hydrated inorganic fillers have found wide acceptance in the wire and cable art. Such compositions are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,442 of North et al. These polymeric compositions exhibit a unique combination, or balance, of improved physical and electrical properties together with a high degree of flame and fire retardance. These highly desirable results are achieved without the use of halogenated polymers such as polyvinyl chloride and chlorosulfonated polyethylene, thereby eliminating hydrogen chloride fumes; without carbon black, thereby permitting its use as colored insulations; without any flame retardant coatings such as are currently required, thereby eliminating an additional step in manufacturing operations when the compositions are used as, e.g., insulating compounds extruded onto a conductor; and without antimony trioxide, thereby eliminating a very expensive compound.
Such compositions find particular use as white (an inherent property) and colored uniinsulation compositions, which can be extruded over metal, e.g., copper or aluminum, conductors, to provide a single layer insulating and jacketing composition which is rated according to U.S. standards for 90.degree. C. operation, and in some cases operation at temperatures as high as 125.degree., at up to 600 volts.
These insulating compositions of North et al. have found particular utility in the insulation of switchboard wire, appliance wire, and automotive wire where a unique combination of superior electrical properties combined with resistance to the degradative effects of heat and flame are essential, and where low smoke density and non-corrosive fumes are desirable.
Besides the three essential components, other additives, such as pigments, stabilizers, lubricants, and antioxidants can be incorporated into the compositions of North et al. Among the antioxidants, polymerized trimethyl dihydro quinoline was found by North et al. to provide effective oxidation inhibition.
In the CSA varnish test, three samples of polymeric insulated wire are (1) heated in an oven, (2) immersed in insulating varnish, (3) reheated in an oven for an extended period, (4) cooled, and (5) bent over a small diameter mandrel. The polymeric composition under test fails if a crack down to the conductor appears in the insulation of any of the three samples when it is bent over the mandrel. Compositions exemplifying the invention of North et al. which employ polymerized trimethyl dihydro quinoline as an antioxidant do not pass the CSA varnish test.
Many polymers are susceptible to oxidation which causes impairment of their physical properties. This degradation may be initiated by heat, light or other energy forms. In most polymers, oxidation proceeds by a free radical chain mechanism. The free radicals form in the polymer under the influence of an internal energy source. These radicals then react with oxygen to form peroxy radical which in turn reacts with the polymer to form a hydroperoxide and another radical which then continues the chain reaction.
Antioxidants have been developed to inhibit polymer degradation. They act either to tie up the peroxy radicals so that these radicals are incapable of propagating the reaction chain, or to decompose the hydroperoxides in such a manner that carbonyl groups and additional free radicals are not formed. The former, called chainbreaking antioxidants, free radical scavengers, or inhibitors, usually are hindered phenols, amines, and the like. The latter, called peroxide decomposers, generally are sulfur compounds (i.e., mercaptans, sulfides, disulfides, sulfoxides, sulfones, thiodipropionic acid esters and the like), or metal complexes of dithiocarbamates and dithiophosphates.
The patent art discloses a number of antioxidants used heretofore with olefinic resins.
U.S. Pat. No. 3,160,680 of Tholstrup et al. and U.S. Pat. No. 3,282,890 of Hagemeyer et al. disclose an antioxidant combination of a sterically hindered phenol and a diester of thiodipropionic acid for use in .alpha.-olefin hydrocarbon polymers. U.S. Pat. No. 3,033,814 of Tholstrup teaches the use of a three component antioxidant consisting of a hindered phenol, a diester of thiopropionic acid and phenyl salicylate in a polymer of a C.sub.2 -C.sub.10 alpha olefin hydrocarbon. U.S. Pat. No. 3,181,971 of Rayner employs the combination of a phenolic antioxidant and a primary or secondary aromatic or aliphatic amino compound with propylene homopolymers or copolymers of propylene with other hydrocarbons. U.S. Pat. No. 3,242,135 of Bown et al. combines an ester of boric acid with a hindered phenol and a diester of thiodipropionic acid to provide oxidation inhibition for homopolymers and copolymers of C.sub.2 -C.sub.8 alpha olefin hydrocarbons. U.S. Pat. No. 3,245,949 of Murdock is directed to homo and copolymers of C.sub.2 -C.sub.8 aliphatic olefin hydrocarbons and mixtures thereof employing as an antioxidant the combination of a phosphorous-containing polyphenolic compound and the dilauryl or distearyl ester of dithiopropionic acid. None of these patents discloses or suggests that the antioxidant combinations can be usefully incorporated in other than hydrocarbon polymers, i.e., no use is suggested with a polymer containing a major amount of an ethylene-vinyl acetate copolymer.
It is an object of this invention to provide a crosslinkable ethylene-vinyl acetate copolymer composition capable of passing the CSA varnish test.
It is another object of this invention to provide an ethylene-vinyl acetate copolymer composition containing silane-treated hydrated inorganic filler which not only exhibits superior moisture and heat resistance and flame retardance but also successfully passes the CSA varnish test.
All percentages and parts expressed in the specification and claims are by weight, unless specifically indicated otherwise.